OpenVPN Client (AirVPN) configuration problem on OpnSense

I have an OpenVPN client configuration problem on OpnSense. I already did post this issue on AirVPN forum.
OpenSense configuration - Troubleshooting and Problems - AirVPN (Link to the AirVPN Forum thread)
Would you care to help me here?

Reading your forum post on the link you provide. it looks like there might be a permission issue on reading the the key file. Was there a password set on the key file you were provided? If so you might have the password wrong.

There is no password set specifically. I coudn’t understand, why this is happening.

I would like to put this question for your convenience here:

How could I translate these lines into OpnSense one-by-one please?

ca "ca.crt"
cert "user.crt"
key "user.key"
remote-cert-tls server
tls-crypt "tls-crypt.key"

I’m not an opnsense guy, I am a pfsense guy sorry I’m not much help on that front. As far as I can tell you config is good as far as the logs say. Your issue is tied to that key file not being read by the client. There is a permission issue somewhere. For testing ONLY you could chmod the file to 777 and see if you can connect, otherwise the file doesn’t exist and it wont work until you get one there with the proper permissions.